This invention relates in general to coating of electrostatographic imaging members and, more specifically, to a process for immersion coating of electrostatographic imaging drums.
Electrostatographic imaging members are well known. Typical electrophotographic imaging members include photosensitive members (photoreceptors) that are commonly utilized in electrophotographic (xerographic) processes in either a flexible belt or a rigid drum configuration. These electrophotographic imaging members comprise a photoconductive layer comprising a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer may be sandwiched between the supporting electrode and a photoconductive layer. Photosensitive members having at least two electrically operative layers, as disclosed above, provide excellent electrostatic latent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a member such as paper.
Electrostatographic imaging drums may be coated by many different techniques such as spraying coating or immersion (dip) coating. Dip coating is a coating method typically involving dipping a substrate in a coating solution and taking up the substrate. In dip coating, the coating thickness depends on the concentration of the coating material and the take-up speed, i.e., the speed of the substrate being lifted from the surface of the coating solution. It is known that the coating thickness generally increases with the coating material concentration and with the take-up speed.
Another technique for immersion coating comprises (a) positioning the substrate within a coating vessel to define a space between the vessel and the substrate and providing a downwardly inclined surface contiguous to the outer surface at the end region of the substrate; (b) filling at least a portion of the space with a coating solution; and (c) withdrawing the coating solution from the space, thereby depositing a layer of the coating solution on the substrate. This process is described in U.S. Pat. No. 5,616,365, the entire disclosure thereof being incorporated herein by reference. When this process is utilized for coating a large drum in which coating fluid is withdrawn at the bottom to deposit a coating layer on the drum located in the center of a coating vessel, it has produced uniform and defect free coating for thin undercoating layers and thick charge transport layers. However, attempts to form a coating of a charge generating dispersion on a previously formed undercoating layer, non-uniform coatings are encountered characterized by fingering patterns and wavy flow patterns throughout the drum surface. These defects are unacceptable for high printing quality requirements such as extremely uniform thickness and defect free charge generating layer coatings. Solutions to these coating problems are crucial for complex, advanced high tolerance imaging systems.